This article examines the main objective and subjective aspects traceable in the concepts of information, document, knowledge, knowledge organization and level of reality. The hypothesis of the author is that objective and subjective aspects are both present in each of these entities, as it is sometimes implicitly recognized even by some of the authors that usually underline exclusively the objective or the subjective elements of these entities. "Moderate constructivism" is the synthesis of objectivism and subjectivism that consists in recognizing that reality is neither completely "given" nor completely "built" and it constitutes (beyond too marked pseudo-oppositions) the paradigm today de facto dominant both in epistemology studies and in knowledge organization studies.